In many rotating machines, seals are provided between the rotating and stationary components. For example, in steam turbines, it is customary to employ seals which have a plurality of arcuate seal ring segments forming, for example, a 360.degree. labyrinth seal, about and between a stationary component (such as a diaphragm) and the turbine rotor. Typically, the arcuate seal ring segments are disposed in an annular groove in the stationary component which is designed to be concentric about the axis of rotation of the machine, and hence concentric to the sealing surface of the rotor. Each arcuate seal segment carries an arcuate seal face in opposition to the sealing surface of the rotor. In labyrinth-type seals, the seal faces carry a radially-directed array of axially spaced teeth, which teeth are radially spaced from an array of axially extending annular grooves forming the sealing surface of the rotor. Alternatively, the rotor may have a smooth surface in radial opposition to the array of teeth on the seal faces. In any event, the sealing function is achieved by creating turbulent flow of the working media, i.e., steam, as it passes through the relatively tight clearances within the tortuous path defined by the seal.
In a typical installation, the annular groove in the diaphragm or other stationary component supporting the seal segments is dovetail-shaped, having opposed, axially oriented locating flanges or hooks defining a slot therebetween. In the exemplary embodiment described herein, the turbine diaphragm is split lengthwise so that the arcuate seal ring segments can be inserted into the respective semi-annular dovetail grooves. More particularly, the arcuate segments have similarly dovetail-shaped mounting portions with a pair of flanges directly axially away from one another for disposition within the dovetail groove, with a radially directed neck joining the seal face and the flanges of the segment, the neck located in the slot defined by the locating flanges of the dovetail groove. The neck thus carries the arcuate seal face radially inwardly of the groove when installed, and the locating flanges of the dovetail groove thus determine the radial position of the seal face vis-a-vis the rotor. The so-called "hook diameter" is measured from the rotor center radially outwardly to the radially outer surfaces of the hooks or locating flanges.
In the case of a steam turbine, it has been found that under normal working conditions, the turbine diaphragms (including those in the high pressure, intermediate pressure, and low pressure sections of the turbine) tend to lose their original round shape and/or specified tolerances. This is especially true in the above mentioned packing ring dovetail groove which, when out of tolerance, can cause unwanted steam leakage, or unwanted rubbing of components, both of which can cause a loss of turbine efficiency.
Welding is typically employed to correct the out of round/out of tolerance problem. The process requires either a weld repair along a locating flange or hook, or in some instances, filling the dovetail in completely. Stress relieving via an appropriate heat treatment method is then required due to the high heat of the welding process. The final step is the remachining of the dovetail to original tolerances. This process is both time consuming and expensive.